Comparison of Large Aperture Scintillometer and Eddy Covariance Measurements: Can Thermal Infrared Data Be Used to Capture Footprint-Induced Differences?

J. C. B. Hoedjes Centre d’Etudes Spatiales de la Biosphère, Toulouse, France

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A. Chehbouni Centre d’Etudes Spatiales de la Biosphère, Toulouse, France

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J. Ezzahar Physics Department LMFE, Faculty of Sciences Semlalia, Marrakech, Morocco

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R. Escadafal Centre d’Etudes Spatiales de la Biosphère, Toulouse, France

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H. A. R. De Bruin Meteorology and Air Quality Group, Wageningen University, Wageningen, Netherlands

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Abstract

Eddy covariance (EC) and large aperture scintillometer (LAS) measurements were collected over an irrigated olive orchard near Marrakech, Morocco. The tall, sparse vegetation in the experimental site was relatively homogeneous, but during irrigation events spatial variability in soil humidity was large. This heterogeneity caused large differences between the source area characteristics of the EC system and the LAS, resulting in a large scatter when comparing sensible heat fluxes obtained from LAS and EC. Radiative surface temperatures were retrieved from thermal infrared satellite images from the Landsat Enhanced Thematical Mapper+ and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellites. Using these images in combination with an analytical footprint model, footprint-weighted radiative surface temperatures for the footprints of the LAS and the EC system were calculated. Comparisons between the difference in measured sensible heat fluxes and the difference in footprint-weighted radiative surface temperature showed that for differences between the footprint-weighted radiative surface temperatures larger than ±0.5 K, correlations with the difference in measured sensible heat flux were good. It was found that radiative surface temperatures, obtained from thermal infrared satellite imagery, can provide a good indication of the spatial variability of soil humidity, and can be used to identify differences between LAS and EC measurements of sensible heat fluxes resulting from this variability.

Corresponding author address: J. C. B. Hoedjes, Centre d’Etudes Spatiales de la Biosphère (CESBIO), 18 Avenue Edouard Belin, 31401 Toulouse CEDEX 9, France. Email: joost.hoedjes@cesbio.cnes.fr

Abstract

Eddy covariance (EC) and large aperture scintillometer (LAS) measurements were collected over an irrigated olive orchard near Marrakech, Morocco. The tall, sparse vegetation in the experimental site was relatively homogeneous, but during irrigation events spatial variability in soil humidity was large. This heterogeneity caused large differences between the source area characteristics of the EC system and the LAS, resulting in a large scatter when comparing sensible heat fluxes obtained from LAS and EC. Radiative surface temperatures were retrieved from thermal infrared satellite images from the Landsat Enhanced Thematical Mapper+ and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellites. Using these images in combination with an analytical footprint model, footprint-weighted radiative surface temperatures for the footprints of the LAS and the EC system were calculated. Comparisons between the difference in measured sensible heat fluxes and the difference in footprint-weighted radiative surface temperature showed that for differences between the footprint-weighted radiative surface temperatures larger than ±0.5 K, correlations with the difference in measured sensible heat flux were good. It was found that radiative surface temperatures, obtained from thermal infrared satellite imagery, can provide a good indication of the spatial variability of soil humidity, and can be used to identify differences between LAS and EC measurements of sensible heat fluxes resulting from this variability.

Corresponding author address: J. C. B. Hoedjes, Centre d’Etudes Spatiales de la Biosphère (CESBIO), 18 Avenue Edouard Belin, 31401 Toulouse CEDEX 9, France. Email: joost.hoedjes@cesbio.cnes.fr

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